Recently, a UWB (ultra wide band) has been attracting attention as new communication means. A UWB enables a large volume of data to be transferred using a wide frequency band in a short distance of approximately 10 m. For example, according to the rules of American FCC (Federal Communication Commission), a frequency band of 3.1 to 10.6 GHz is planned to be used. In this manner, the UWB is characterized by using a very wide frequency band.
Recently, studies on a bandpass filter having a very wide pass band that can be used for such a UWB have been extensively performed. For example, it is reported that a very wide pass band having a pass bandwidth in which the fractional bandwidth (bandwidth/center frequency) is more than 100% can be obtained using a bandpass filter to which the principles of a directional coupler have been applied (see a non-patent document “Ultra-wide Bandpass Filter Using Microstrip-CPW Broadside Coupling Structure”, March, 2005, Collection of Papers Presented at General Conference of the Institute of Electronics, Information and Communication Engineers, C-2-114 p. 147, for example).
Meanwhile, as a widely used conventional bandpass filter, a configuration is known in which a plurality of quarter-wavelength stripline resonators are arranged side by side and coupled to each other (see Japanese Unexamined Patent Publication JP-A 2004-180032, for example).
However, both bandpass filters proposed in the above-described non-patent document and JP-A 2004-180032 are problematic, and are not suitable for the use for a UWB.
For example, the bandpass filter proposed in the above-described non-patent document is problematic in that the pass bandwidth is too wide. That is to say, a UWB basically uses a frequency band of 3.1 GHz to 10.6 GHz, but International Telecommunications Union, Radio Communications Sector sets up a standard in which the band is divided into a low band that uses a frequency band of approximately 3.1 to 4.7 GHz and a high band that uses a frequency band of approximately 6 GHz to 10.6 GHz so as to avoid 5.3 GHz used by IEEE802.11.a. Thus, each of a low band filter that passes signals in the low band and a high band filter that passes signals in the high band is required to have a pass bandwidth in which the fractional bandwidth is approximately 40% to 50% and to have an attenuation at 5.3 GHz, and, thus, the bandpass filter proposed in the above-described non-patent document having a pass bandwidth in which the fractional bandwidth is more than 100% cannot be used because the pass bandwidth is too wide.
Furthermore, the pass bandwidth of a conventional bandpass filter using ¼ wavelength resonators is too narrow, and, even in the pass bandwidth of the bandpass filter described in JP-A 2004-180032, which has been adjusted so as to have a wider band, the fractional bandwidth is less than 10%. Thus, this filter cannot be used as a UWB bandpass filter that is required to have a wide pass bandwidth corresponding to a fractional bandwidth of 40% to 50%.
Moreover, in the case where both of the low band and the high band are used, in a RF IC that processes high frequency signals, a circuit that processes signals in the low band and a circuit that processes signals in the high band are different from each other, and, thus, two terminals may be provided on the antenna side, and there is increasing need for a diplexer that connects a low band-side terminal and a high band-side terminal, and an antenna.